1. Field of the Invention
The present invention relates to relay circuits and, in particular, to safety relay control circuits that are employed to monitor the status of power contactors.
2. Background of the Invention
In many industrial and other systems, high levels of power are required by loads, and must be repeatedly switched on and off with respect to the loads. Power contactors are commonly employed to provide this switching function. Although rare, it is occasionally possible that one or more of the contacts within such power contactors, which are normally open, become welded closed due to arcing that occurs during the switching process. Because such welding of one or more contacts precludes those contacts from being switched off, and thus precludes those contacts from being employed to switch off the power to the loads, it is important that welded contacts be identified soon after a welding event occurs so that the welded contacts can be replaced.
In order to identify welded contacts, safety relay circuits can be employed. Referring to FIG. 1, a Prior Art safety relay circuit 100 is shown that is capable of monitoring whether any of three power contacts 102, 104 and 106 have become welded closed. As shown, each of the three power contacts 102, 104 and 106 governs whether a corresponding single-phase of power provided by a respective line 112, 114 and 116 is provided to a high-power, three-phase load 108. The safety relay circuit 100, upon identifying whether any of the three power contacts 102, 104 or 106 is welded closed, provides a signal to an operator (or other controller or control system) indicating the welded state of the power contact(s), typically by preventing the operator from restarting a system 110 of which the three-phase load 108 and the safety relay circuit 100 form a part.
The safety relay circuit 100, which is shown in a ladder diagram format in FIG. 1, operates as follows. As shown, an operator can press an ON button 118 so that a first port 124 of a standard safety relay 120 is coupled to a power (or voltage or current) source 122. In the present embodiment, the standard safety relay 120 is an A-B 440R-ZBL220224 safety relay manufactured by the Allen-Bradley Company of Milwaukee, Wis. In alternate embodiments, the safety relay 120 can be another type of safety relay. In the embodiment shown, the safety relay 120 is configured to apply a voltage to a second port 126 once the voltage source 122 is coupled to the first port 124.
The safety relay circuit 100 further includes a normally-closed auxiliary contact 130 coupled between the second port 126 and a third port 128 of the safety relay 120. Assuming that the auxiliary contact 130 is in fact in its closed position, the third port 128 experiences a voltage change as the voltage is applied by the safety relay 120 to the second port 126. The safety relay 120 is further coupled to ground 123 at a fourth port 132. If each of the above-described events occurs, the safety relay 120 switches from an off state to an on state. Also as shown, the safety relay 120 in this embodiment includes several other ports, including ports 125, 127, 129 and 131, between which are coupled a set of emergency stop switches 133, which allow for the safety relay to be switched to its off state in an emergency. Further, the safety relay circuit 100 is coupled to certain of the lines 114,116 by way of a power transformer 135, one side of which is coupled between the power source 122 and ground 123, and the safety relay circuit further includes one or more protective fuses 137 that can have a variety of current tolerances.
As shown on line 5 of the ladder diagram, the safety relay 120 has fifth and sixth ports 140 and 134, respectively, between which are coupled first and second normally-open contacts 136 and 138, respectively. The fifth port 140 is additionally coupled to the power source 122 while the sixth port 134 is additionally coupled to a first coil 142, which is coupled between the sixth port and the ground 123. In accordance with the ladder diagram, the first and second normally-open contacts 136,138 within the safety relay 120 close when the safety relay switches to its on state, such that the power source 122 is electrically coupled to the first coil 142. The first coil 142, which is thus coupled between the voltage source 122 and ground, consequently conducts and is energized.
When the first coil 142 is energized, several contacts switch from their normal positions to their excited positions. First, each of the power contacts 102-106 switches to its closed position, such that the three single-phases of power associated with the lines 112-116 are provided to the three-phase load 108. Additionally, in accordance with the ladder diagram, the normally-closed auxiliary contact 130 is opened and a normally-open sustaining contact 143 that is coupled in parallel with the ON button 118 is closed. The sustaining contact 143 thus maintains the electrical coupling between the first port 124 and the power source 122 even though the operator releases the ON button 118.
The safety relay 120 is designed to operate so that, as long as the voltage source 122 continues to be coupled to the safety relay 120, the safety relay remains in its on state such that the contacts 136,138 remain closed and the first coil 142 remains conductive, even though the auxiliary contact 130 is opened. That is, the auxiliary contact 130 need only be closed at the time that the first port 124 is coupled to the voltage source 122 in order for the safety relay 120 to enter its on state, and need not remain closed thereafter in order for the safety relay 120 to remain in its on state. In order to deenergize the first coil 142, and consequently to open the power contacts 102-106, open the sustaining contact 143 and close the auxiliary contact 130, the operator presses an OFF button 144. This causes a decoupling of the power source 122 from the first port 124, which causes the safety relay 120 to return to an off state so that both of the contacts 136,138 open.
The safety relay circuit 100 is usually capable of providing an indication to an operator when one or more of the power contacts 102-106 has welded and will not return to an open position. As shown in FIG. 2 (Prior Art), each of the power contacts 102-106 and the auxiliary contact 130 are physically positioned within a single power contactor 146 along with the coil 142. All of the power contacts 102-106 and the auxiliary contact 130 are physically coupled to one another so that, during proper operation, the contacts are all in their normal positions, in their excited positions, or in between their normal and excited positions.
Usually, when the first coil 142 becomes conductive or energized, all of the contacts 102-106 and 130 move to their excited positions. Also, the power contactor 146 is spring-loaded such that, when the first coil 142 becomes non-conductive or deenergized, the contacts 102-106 and 130 usually all return to their normal positions. However, if any one or more of the power contacts 102-106 become welded in their respective closed positions, the spring-loading within the power contactor 146 is insufficient to cause the contacts to return to their normal positions. That is, one of the power contacts 102-106 becomes locked in its excited, closed position, while the auxiliary contact 130 becomes locked in its excited, open position.
Because welding of any of the power contacts 102-106 causes the auxiliary contact 130 to become locked in its open position, the safety relay circuit 100 is able to provide the operator with an indication that a welding event has occurred. Specifically, because the auxiliary contact 130 is locked in its open position, the safety relay 120 does not experience a voltage change at its third port 128 when the ON button 118 is pressed and the power source 122 is coupled to the first port 124. Consequently, once the system 110 is shut down due to the pressing of the OFF button 144 and the deenergizing of the coil 142, the operator is later not able to restart the system by pressing the ON button 118.
Although the safety relay circuit 100 as implemented in conjunction with the power contactor 146 is capable of providing an indication of when one or more of the power contacts 102-106 have experienced a welding event, the safety relay circuit and power contactor 146 could be improved in two respects. First, as shown in FIG. 2, the physical layout of the power contactor 146 can be such that the distances between the auxiliary contact 130 and each of the respective power contacts 102-106 are unequal and/or significant. Given such a physical layout, it is possible that under certain physical stresses the power contactor 146 could be twisted or bent in such a manner as to eliminate or hinder the physical coupling of one or more of the physical contacts 102-106 with the auxiliary contact 130.
In particular, the physical coupling between the power contact 102, which is the power contact farthest from the auxiliary contact 130, and the remaining contacts 104,106,130 could be significantly undermined. If this happened, and the power contact 102 was welded in its closed position, then it would be possible that the remaining power contacts 104,106 would return to their normal, open positions and the auxiliary contact 130 would follow those remaining power contacts and return to its normal, closed position. In such a circumstance, the safety relay circuit 100 would not be able to provide an indication that a welding event had occurred with respect to the power contact 102, since the auxiliary contact 130 would be in its normal, closed position when the ON button 118 was pressed.
Second, although the safety relay circuit 100 is normally capable of providing, in the absence of the above-described physical damage to the power contactor 146, an indication that a welding event has occurred when any of the power contacts 102-106 have welded, the safety relay circuit 100 is not capable of providing such an indication in the circumstance where the auxiliary contact 130 itself has welded in its closed position. Despite the physical coupling of the auxiliary contact 130 to the power contacts 102-106, the physical size and structural strength of the power contacts are significantly greater than that of the auxiliary contact, such that the welding of the auxiliary contact 130 in its closed position does not physically restrict the motion of the power contacts. Rather, when the auxiliary contact 130 is welded, the physical connection between the auxiliary contact and the power contacts 102-106 will tend to break or diminish as the power contacts attempt to move to their closed positions.
Consequently, if the auxiliary contact 130 is welded, the pressing of the ON button 118 can continue to cause the closing of the power contacts 102-106 and start the system 110, as if nothing had happened. At the same time, since the auxiliary contact 130 cannot open upon the welding of any of the power contacts 102-106, the safety relay circuit 100 also continues to allow starting of the system 110 even when one or more of the power contacts are additionally welded shut.
For these reasons, a need exists for an improved safety relay circuit. In particular, a need exists for a safety relay circuit that will successfully indicate the welding of a power contact of a power contactor even when the power contactor was significantly physically bent or twisted. Additionally, a need exists for a safety relay circuit that will not only indicate when a power contact has been welded, but also will indicate when a welding problem has occurred with respect to an auxiliary contact employed by the safety relay circuit in monitoring the power contacts.
The present inventors have discovered a safety relay circuit that is capable of monitoring whether one or more main contacts of a contactor have welded, and also capable of determining whether one or more auxiliary contacts have welded. In one embodiment, the safety relay circuit includes four circuits, the first of which has an on state in which a coil of the contactor is energized, and an off state in which the coil is deenergized. The first circuit switches from the off state to the on state when a first signal (such as power from a power source) is provided to the first circuit if an additional signal is also present, and then remains in the on state until the first signal is discontinued. The second circuit prevents the additional signal from being received by the first circuit when the auxiliary contact is in a position indicating that one of the main contacts is welded. The third circuit causes the first signal to be discontinued when the fourth circuit provides a further signal indicating that one of the auxiliary contacts is welded. The further signal is provided when both the coil is energized and yet the auxiliary contact remains in its normal position. A time delay feature is present in the third circuit so that the further signal indicating that the auxiliary contact has welded must be provided for a certain time before the third circuit causes the first signal to be discontinued.
In particular, the present invention relates to an apparatus for monitoring a first main contact within a first contactor, where the first contactor further includes a first auxiliary contact and a first primary coil, and where energizing of the first primary coil by way of a power source causes each of the first main contact and the first auxiliary contact to switch. The apparatus includes a trigger coupled to a power terminal capable of being coupled to the power source, and a first circuit that is electrically connected to the power terminal upon actuation of the trigger and that has an on state and an off state. The first circuit switches from the off state to the on state when the power source is electrically connected to the first circuit by way of the power terminal if an additional signal is received by the first circuit at that time. Also, the first circuit remains in the on state once it has entered that state until the power source is disconnected from the first circuit, and the first circuit causes the primary coil to be energized when the first circuit is in the on state. The apparatus additionally includes a second circuit coupled to the first circuit and to the auxiliary contact, where the second circuit prevents the additional signal from being received by the first circuit when the auxiliary contact is in a first position, and a third circuit that causes the first circuit to be electrically disconnected from the power terminal when the third circuit is in a first state, and further causes the first circuit to be electrically connected to the power terminal while the third circuit is in a second state. The apparatus further includes a fourth circuit coupled to the third circuit and including portions of the first and second circuits. The fourth circuit causes the third circuit to enter the first state when at least one of the first circuit enters the off state such that the primary coil becomes and remains deenergized for a first period of time, and the first auxiliary contact remains in a second position for the first period of time while the first circuit is in the on state.
The present invention further relates to an apparatus for monitoring a main contact within a power contactor, where the power contactor further includes an auxiliary contact and a coil. The apparatus includes means for controlling an energizing of the coil, where the coil becomes energized only when the means for controlling receives both a first signal and a second signal, and where the coil remains energized as long as the means for controlling continues to receive the first signal. The apparatus further includes means for determining whether the second signal is provided to the means for controlling based upon a status of the auxiliary contact. The apparatus additionally includes means for preventing the first signal from continuing to be provided to the means for controlling when it is determined that, despite the energizing of the coil, the status of the auxiliary contact has not changed.
The present invention additionally relates to, in a system for monitoring a main contact of a contactor that includes a primary coil and a first auxiliary contact, a method of monitoring the first auxiliary contact employed in monitoring the main contact. The method includes providing a first signal to a first circuit of the system, providing a second signal from a second circuit of the system to the first circuit if the second circuit determines that the auxiliary contact is in a first position, and switching the first circuit from an off state to an on state upon receiving the first and second signals at the first circuit, where in response the primary coil is energized. The method further includes providing a third signal to a third circuit of the system when, after the energizing of the primary coil, the auxiliary contact remains in the first position, and causing the first signal to no longer be provided to the first circuit in response to the third signal so that the first circuit returns to the off state and the primary coil is deenergized.
The present invention further relates to a power contactor that includes a coil having first and second ends and first and second sides connecting the first and second ends, a first auxiliary contact supported by the power contactor along the first side of the coil, and a second auxiliary contact supported by the power contactor along the second side of the coil. The power contactor further includes a first power contact supported by the power contactor proximate the first end of the coil nearer to the first auxiliary contact than to the second auxiliary contact, and a second power contact supported by the power contactor proximate the first end of the coil nearer to the second auxiliary contact than to the first auxiliary contact. Each of the auxiliary contacts and power contacts are configured to switch from their respective normal statuses to their respective alternate statuses upon energizing of the coil.